Also, since RDJ is middle aged, it would be weird to have a Steve who was significantly younger than him (like Kellan Lutz) and would maybe draw too much attention to RDJ's age as he gets older (if he does a few of the movies *crosses fingers*).

This is a major concern for me too. It will also be tricky because it's canon that Tony's a love-sick puppy maaajor fanboy of Captain America. Whoever they cast needs to be on par with Jr. (Well... no one is on par with Jr. because Robert is an Ascended Fanboy God, but, you get what I mean!) :)